Controlling directivity of loudspeakers has always been one of the most important problems in commercial sound reproduction. Being able to aim and deliver sound energy precisely to one area and prevent the sound energy from falling onto another is one of the challenges of sound system designers. A speaker system with well controlled directivity will have precise and even SPL (sound pressure level) coverage of the audience area, providing desired speech intelligibility and balanced reproduction. In addition, controlled directivity helps avoid reflective surfaces, insuring controlled reverberation and minimum interference with direct sound.
Various devices have been used to control sound dispersion. The horn is one of the methods that is quite effective for mid and high frequency transducers. Another effective method that is known from antenna theory is using multiple drivers arranged in a line source or array.
The directive properties of such line sources or arrays are known. If transducers are spaced very close to each other in long line with a length that is comparable to or larger than a wavelength of radiated sound, then such a system generally exhibits rather directive properties on its axis and would project sound without wasting too much energy on off-axis radiation.
A line array system concept is derived from line source theory. An ideal line source is an infinite, thin (narrow) and continuous vibrating element, which radiates cylindrical waves. Such a line source has an important radiation property, which is that its SPL level decreases inversely proportionately to the distance from the source, losing only 3 dB with each doubling of the distance. A point source radiator (common loudspeakers are considered to be point source radiators) generates a spherical wave. Its SPL decreases inversely proportionately to the square of the distance from the source, losing 6 dB with each doubling of the distance.
This phenomenon can be understood, considering that expansion of a cylindrical wavefront results in a surface area gain being proportionate to increasing distance, while expansion of a spherical wavefront produces an area gain, which is proportionate to the square of the distance.
Unlike the infinite ideal line radiator, a line source with limited length has limited extension of its cylindrical wavefront zone (near field). Beyond a certain distance, the cylindrical wavefront gradually transforms into the spherical wavefront (far field) and the system becomes a point source device. The distance, defining a border between the near and far field zones, depends on the line source length and frequency. Generally, the near field extends from the line source to a distance D=L2f/636, where L is the length of the line array, and f is the relevant frequency. Within the near field the SPL loss is about 3 dB and the intensity is proportional to about l/r, where is the listener's distance from the line source. In the far field, the SPL loss is about 6 dB, and the intensity is proportional to l/r2.
The benefits of a line source in comparison to a point source system can be stated as follows. First, a significantly smaller SPL reduction with distance allows for delivering higher sound volume levels further to the audience. Second, at a given sound level at the back of a venue a line source would produce much smaller difference in SPL levels throughout the venue, with SPL being significantly lower in close proximity to the source. This provides very comfortable listening conditions without the danger of overpowering the audience in the front rows. Third, the cylindrical wavefront provides very controlled energy dispersion in the plane, which coincides with the line source (in most applications this would be the vertical plane), resulting in excellent intelligibility even in a very reverberant environment.
Multiple transducers have been used to build column speakers to deliver direct sound further to the audience. However the problem is that conventional transducers or compression drivers do not work well in such applications at high frequencies. A true line array system is different from a line source in that it consists of a discrete array of transducers and has limited length. In this case, the notion of a continuous line source should be considered in the relationship between line array geometry and the wavelength of reproduced sound. The primary question defining the proper operation of a line array is whether the array can be considered as a continuous line source over the reproduced frequency range.
Consider a line array system comprising a number of linearly placed, spaced apart drivers on a length L, where P is an equivalent radiating piston height (diameter for a circular piston) of a driver, and H is a space taken by each driver or distance between driver centers. The condition that defines a discrete line array as a line source can be related to two different shapes of the radiating element. For circular drivers, proper line source behavior, or “coupling”, can be achieved in a frequency range where:H<λ,
where λ is a wavelength at a given frequency.
For example, to fulfill this condition at 10 kHz and above, drivers must be spaced with less than 1.33″ (3.4 cm) between driver centers.
This spacing requirement is a completely unrealistic condition for a practical design using 1″ or even 0.75″ dome tweeters, which would obviously require a greater spacing. This means that conventional line arrays using conventional cone drivers cannot properly perform as a line source at high frequencies. If a line array is not properly coupled, the resulting dispersion is far from consistent and exhibits severe lobing along with significant SPL irregularities within the coverage area.
As noted above, a line source system where drivers are positioned in a line and are driven with the same signal possesses very narrow vertical dispersion. Depending on the system's length and frequency, the vertical dispersion beyond the array's upper and lower limits approaches zero degrees. This means that a straight array radiates sound strictly between upper and lower planes limited with its physical dimensions. In applications where the audience is located on a flat floor such configuration is acceptable.
However, for many environments (e.g., large halls, venues, etc.) such controlled dispersion is a disadvantage since the audience may be located on elevated/tiered floor with height gradient M at the back. Some conventional solutions simply try to use more straight columns. In order for a system of straight columns to cover the audience in such venue it would be necessary to install a straight system of the exact height M. This increases the cost and complexity of such an installation. Another existing option is to use discrete arrays where each driver set is installed in a physically separate box and all the boxes are connected through elaborate system of hinges and pins; the boxes are then usually mounted on a special bumper bar, and metal chains and motors are used to attach the speaker array to the ceiling. Such installations tend to be prohibitively expensive for many applications, and certainly is not capable of being wall mounted in most medium size venues.